Normal brain functioning depends on the establishment of diverse cellular phenotypes. Our research program addresses the mechanisms that regulate differentiation of neuroendocrine cells during development. We use Drosophila molecular genetics to examine the biology of a basic HLH protein called DIMMED (DIMM). Our genetic studies indicate DIMM is necessary and sufficient to promote the cellular features that define neuroendocrine differentiation. First, DIMM is normally expressed by most of the principal peptidergic cells in Drosophila (though not all). Loss-of-function analysis indicates DIMM is not required for cell survival or growth, but is required for these cells to display a proper regulated secretory pathway and normal accumulation of neuropeptides. Gain-of-function studies indicate DIMM can activate the cellular machinery to support peptidergic function in non-peptidergic (i.e., conventional) neurons. At a molecular level, we have shown that DIMM is a transcription factor and that it activates several specific target genes. DIMM is related to the mammalian protein Mist1, a factor implicated in secretory cell differentiation. We hypothesize that DIMM is a dedicated, pro-secretory factor with conserved functions, whose study will lead to a better understanding of the organization and differentiation of neuroendocrine cell types. This research program supports efforts to address human syndromes caused by underlying neuroendocrine disorders, such as stress, or tumor formation in the pituitary or pancreas, and will help guide future programs of stem cell differentiation to generate specific neuroendocrine lineages in vitro. Here we propose five specific aims. First we will derive a consensus DIMM cis-binding site by analyzing in vitro and in vivo several direct target genes. Second, we will extend the list of molecular targets. Third, we will examine DIMM action mechanisms by genetically testing the roles of individual DIMM targets. Fourth, we will test the hypothesis that the paired homeoprotein Eyeless is the principal activator of dimmed. Finally, we will extend our analysis to the mouse model system by examining the hypothesis that the DIMM orthologue Mist1 plays an important role in mammalian peptidergic cell biology.